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The Planetary Society Blog

By Emily Lakdawalla

Here we go!

Feb. 23, 2007 | 16:15 PST | Feb. 24 00:15 UTC

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I'm delighted to have the following update on the New Horizons Jupiter flyby from John Spencer. --ESL

by John Spencer

Whoa, here it comes. As I write, we (of course I mean New Horizons, but it always feels like "us" up there on the spacecraft) are now a mere 7.8 million kilometers from Jupiter, just over three times our distance at closest approach, which happens next Wednesday. Other than gathering plasma data, the spacecraft is taking a breather right now (we had reserved this time for a trajectory correction maneuver, which we canceled a while ago because our trajectory needs no stinkin' corrections), but six hours from now [or about the time this blog was posted --ESL], at 00:40 Universal Time on the 24th, our baby leaps into action with the first of its many observations of Io. From that moment until March 4th, well into the outbound leg of our journey, the slews, the scans, the stares, the integrations come thick and fast..

We have actually completed our first observation of the close encounter period, which we snuck into yesterday's timeline after the trajectory correction maneuver was cancelled. On Thursday, Alice, our ultraviolet instrument, watched a star pass behind Jupiter's atmosphere and re-emerge on the other side a few hours later, in order to probe the planet's upper atmosphere. We won't get the full data down for a few weeks, so I (having not paid attention to the details of our real-time data stream) expected no more than an "observation successful" signal from the spacecraft today. But it's so much better than that. Here's what appeared in our mailboxes this morning from Maarten Versteeg of the Alice team:

Stellar occultation by Jupiter observed by New Horizons Alice

Click to enlarge >

Credit: Maarten Versteeg, Southwest Research Institute, and New Horizons

This is a graph of the total brightness seen by the instrument as the observation progressed. Alice slews to the star, opens its door, and watches the signal gradually increase as the star drifts closer to Jupiter and the planet itself intrudes into the aperture. Then suddenly the signal drops as the star winks out behind Jupiter. After a decent interval Alice closes its door and waits for the process to repeat, in reverse, when the star re-emerges. The science payoff is in the details of that sudden drop and re-emergence, which we won't see until the full data come down, but for now this plot is wonderful confirmation that (a) we successfully pointed at the star; (b) the star had the expected brightness, so it was well centered in the aperture and the instrument sensitivity is what we predicted; and (c) the occultation occurred at the time predicted, right in the middle of our observations (no surprise -- but lovely to see in any case). The first close-encounter observation is in the bag!

There was more excitement yesterday. We've been turning our Earthly (and near-Earthly) telescopes on the Jupiter system too, to give us the broader context for New Horizons' snapshot of the system. Yesterday Kandis Lea Jessup and I got our first look at the first Hubble pictures of Io, taken back on Valentine's Day (these were the images we had to scramble to redesign at the end of January, after the failure of Hubble's other camera). Io was only 16 pixels across in the pictures, but that was enough to show us something very interesting at ultraviolet wavelengths: there was a huge volcanic plume rising above the edge of Io's disk. We're not yet 100% sure which volcano is generating the plume, but I have a hunch that it's Tvashtar, a volcano that obliged Cassini by producing a similar-sized plume during Cassini's Jupiter flyby in late 2000. We've seen plumes like this in Hubble images before, but they aren't particularly common, so we are excited at the prospect of getting much closer images of this thing with New Horizons next week. In fact the New Horizons schedule includes a color image specifically to look at Tvashtar's plume, on the off-chance that there might be something there to see. We just need Tvashtar (or whatever volcano is actually responsible) to keep doing its stuff for one more week.

Io from Hubble on Valentine's Day
The left image, in approximately true color, shows the orange oval deposits of the Pele volcano, and other familiar albedo features. The ultraviolet image on the right shows a big plume (arrowed) rising above the surface not far from the north pole. Credit: John Spencer and Kandis Lea Jessup, Southwest Research Institute, and the Space Telescope Science Institute

Next week most of the New Horizons team will converge at the Johns Hopkins Applied Physics Laboratory in Maryland, where the spacecraft was built, for a science team meeting. There won't be much Jupiter data to discuss yet, but we do plan on sending down a few images during the close approach phase, as appetizers. First down, on Monday, will be one of our approach Io images, about 180 pixels across compared to those 16-pixel Hubble images. If we're really lucky, we might see the big plume in that first picture.

This is all pretty thrilling. We are literally gathering momentum as we fall into Jupiter's gravitational well, and it feels like that too -- the exhilarating, scary, rush of a downward swoop on a roller-coaster. All that planning is about to pay off, and there's nothing to do but enjoy the ride. And now we're down to 7.6 million kilometers from Jupiter...

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